I have the MMT in Aygo and I presume the function is the same as your ASG.
On level ground, park your car, bring the lever to neutral and apply handbrake. Shut off your car, remove key.
On inclined ground, park your car, leave the gear in D or R (D if it is parked uphill and R if it is parked down hill), apply handbrake and then shut off the engine, remove key. Here you have two line of defence to stop the car rolling, i.e. firstly the gear that is engaged (D or R) and secondly the hand brake.
When you have parked your car in gear and want to restart the engine and drive, the car will not start if you do not bring the gear to neutral. The gear can not be put to neutral if you do not insert and turn the key two clicks. So begin by inserting your key first and then turn your key two clicks, bring your gear to neutral (the car will not roll down because your hand brake is still on) , press your foot brake and start the car. Release the hand brake and pull out. I suppose you all have a hill assist function, making it easy to drive away from incline.
There are many posts in Aygo forum there people have parked the car in gear and have been unable to start beacuse they have forgotten to bring back the gear to neutral by inserting the key and turning it partially to bring the gear to neutral first before you can start the car.